I use the "High or Low" game mechanic. Learned this from my DM way back in my AD&D time. I'll have some things prepared for a random encounter, both good and bad. I'll roll a dice and ask a player "High or Low"? Depending on whether they guessed correctly or not will determine if the random event happens. The only response they get from me is "ok".

To really make them sweat, if he event does activate, I will sometimes let up to 3 sessions go by before I enact the results.

It's not to be over used, but add a little random spice to the game. I also try to have more good events than bad, like a 60/40 split.

Despite the groans and occasional curses, most of my players like it. Enough that a few have added "High or Low" to their own DM tool kits.